The European Commission has launched a formal investigation of Microsoft's compliance with commitments it made to settle a browser antitrust case.

In 2009, the company agreed to offer Windows users in the European Union a simple way to pick their preferred default web browser, rather than forcing them to use Internet Explorer. But now the Commission believes that Microsoft did not include the browser choice mechanism in Windows 7 Service Pack 1, released in February 2011, despite Microsoft claiming in a December 2011 report that it had complied with its commitments.

Millions of Windows users in the E.U. may not have been offered a choice of default browser as a result, the Commission said on Tuesday.

Microsoft said in a statement that it had fallen short in its responsibility to offer the browser choice to all Windows users.

A technical error meant that it failed to offer the browser choice on PCs that came with the service pack 1 update to Windows 7, the company said. However, the choice screen was delivered correctly to PCs running the original version of Windows 7, and to the relevant versions of Windows XP and Windows Vista, it said.

When the company heard from the Commission on July 2 that it had not distributed the browser choice software to roughly 28 million PCs running Windows 7 SP1, it immediately began testing a fix and began distributing it the following day, it said.

Outside counsel have investigated how the problem occurred and will report to the Commission shortly, the company said.

Microsoft made a legally binding commitment to offer the browser choice through December 2014, in settlement of a case in which it was accused of abusing its dominant position in the market for PC operating systems by tying its browser to Windows.

Microsoft estimated that the browser choice software was received by 90 percent of the PCs that should have received it.

The Commission now plans to investigate whether Microsoft failed to comply with its commitments, in particular the commitment to provide a browser choice screen to Windows 7 users. If the company has breached its commitments, it could be fined up to 10 percent of annual revenue.

The company has offered to extend the period for which it must comply with the 2009 ruling by 15 months.

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